Precision current sources are normally generated with a reference voltage applied across a precision resistor which provides a current to a tolerance proportional to the tolerance of the voltage and the resistance. In some integrated circuit technologies, for example CMOS technology, resistors can be fabricated but with insufficient control to obtain precise predictable resistance values. Additionally, such resistors vary significantly with environmental changes such as temperature. In comparision, capacitors can be fabricated with substantially more precision than resistors. Using capacitors, however, is not convenient for generating current sources, particularly d.c. current sources.